The OS is a killer mill no doubt ! from running them both I can say they are very competitive with each other, and each one has its own strengths over the other... The B5 is punchier, which I personally prefer..its still very smooth but it has more snap in the trigger then the OS does...The OS is very smooth and little less responsive on the trigger..tho its not lagging or anything like that, but its just not quite as crisp on trigger as the B5... The B5 appears to be getting a little better runtimes, but we are running the B5 with a little less crank timing then the OS , as well we were running the OS on a 8 mm venturi and the B5 on a 7mm venturi....On the dyno I ran the OS on a 8 mm and the B5 on a 7 mm....Both engines tune very easy, they both rip and either one would make me happy....So whichever one floats your boat, you really can't go wrong with either IMO